your local stereo shop should carry one, even a cheapy type one by scosche or w/e will work. if you can use balanced signal, depends on amp. also you will be throwing higher voltage at the amp than some are designed to take, plus balanced runs basically opposite phase of each other and are meant to then be summed up then the opposite sound waves cancel each other out. not all amps can take this signal, some can but are not designed for it so you get crappy results, some cant take 8+ volts input and will live a short life, throw overload protection, thermal protection ect ect ect. when in doubt it never hurts to use one. any place near you that sells car audio should be able to help you out, but really even best buy(did you talk to sales rep or install bay rep?) should carry a cheapy type one.http://www.crutchfield.com/p_142SLC4...ine+converters
theres a link to a cheapy one in case you dont know what you should be looking for.